The present invention is directed to a method for temporarily protecting glass articles, in particular for protecting the surfaces of glass articles such as Liquid Crystal Display (LCD) glass. The method is useful, for example, for protecting glass articles from ambient contaminants and for preventing glass chip adhesions during cutting or grinding of the glass article.
Many uses of glass, including LCD glass, require a very clean glass surface that is substantially free of dust and organic contaminants. When exposed to the environment, glass can quickly become contaminated with organic contaminants, with contamination being observed within a few minutes. Cleaning processes currently used for cleaning LCD glass often involve several steps and require a variety of chemicals. There is a need, therefore, for a method of protecting a glass surface from ambient dirt during manufacture, shipping and storage to minimize or even eliminate the need for chemicals to provide a clean glass surface.
Current procedures used to cut and grind glass surfaces and edges often generate small glass chips (e.g., chips having a size greater than 1 micron and less than about 100 microns). Some of these particles irreversibly adhere to the clean glass surface, rendering the glass useless for most applications. This is particularly a serious problem in the case of LCD glass surfaces.
LCD glass can be made by a fusion draw process, which yields flat, smooth glass surfaces which can be cut or ground to the desired size. Some of the glass chips generated from the cutting process originate from the surface of the glass. When the flat surface of these chips comes into contact with the surface of the glass plate, there can be a large contact area between the chips and the glass surface which promotes strong adhesion. If a water film condenses between these two surfaces, permanent chemical bonding may occur, in which case the adhesion of the glass chips to the surface becomes irreversible. This may make the glass useless for LCD applications.
One known method of protecting glass sheets, specifically, sheets of LCD glass, is to apply a polymer film on both major surfaces of the glass to protect the glass during the scoring, breaking and beveling processes. In a typical method, one major surface has a polymer film attached with an adhesive, and the other major surface has a film attached by static charge. The first film is removed after the edge finishing (cutting or grinding) of the sheet is completed, while the second is removed prior to the finishing process. Although the adhesive-backed film protects the surface from scratching by the handling equipment, it causes other problems. For example, the polymer may entrap glass chips produced during the finishing process, leading to a build up of glass chips and scratching of the glass surface, particularly near the edges of the surface. Another problem with this film is that it may leave an adhesive residue on the glass surface. There is a need, therefore, for a method of protecting a glass surface from chip adhesions that does not leave any residual coating on the glass surface, and for a method of temporarily protecting glass surfaces, whereby a glass article with a clean, coating-free surface can be readily obtained for further use.
Organic coatings have been used to protect glass surfaces for many years. See, for example, Smay, G. L. Glass Technology 1985, 26, 46-59. Often oleic acid or stearic acid solutions are applied to create a lubricious coating allowing glass bottles to slide among each other without generating flaws or cracks in the manufacturing process. Oleic and stearic acids have long aliphatic chains which render them relatively insoluble in water. Therefore, an alcohol/water solution is required if, for example, the lubricating layer is to be applied by spraying. In terms of the manufacturing of glass and, in particular, LCD glass, the alcohol in the alcohol/water solution is a serious problem since when sprayed on a hot glass surface the alcohol will rapidly evaporate and can produce a flammable and potentially explosive mixture of air and alcohol vapor.
Oleic and stearic acid coatings are also unsuitable for use in protecting LCD glass because their chain-chain interactions are very strong. As a result, the coatings are difficult to remove even in a basic, detergent wash.
A critical aspect of any coating used to temporarily protect LCD glass is removability. Manufacturers of liquid crystal displays use LCD glass as the starting point for complex manufacturing processes which typically involve forming semiconductor devices, e.g., thin film transistors, on the glass substrate. To not adversely affect such processes, any coating used to protect LCD glass must be readily removable prior to the beginning of the LCD production process.
Some materials, such as silanes and siloxanes, may be able to satisfy the removability criterion in a qualitative sense, i.e., under suitable conditions they can be substantially completely removed from a glass surface, and yet the materials are still unacceptable for use with LCD glass based on this very same criterion. This is because manufacturers of liquid crystal displays consider these materials so potentially disruptive of the LCD production process that even the most minimal residual levels of the materials are not considered acceptable. Accordingly, although basically removable, these materials do not satisfy the removability criterion as applied in practice (i.e., the level of removability which a protective coating must achieve is a function of the composition of the coating).
Another class of materials which needs to be avoided in connection with glasses to be used to make liquid crystal displays are those that contain alkalis. This is because alkalis, even in small amounts, are known to poison thin film transistors. Similarly, metals, especially heavy metals, are undesirable since they can change the electrical properties of the glass surface.
Like the oleic and stearic acid coatings discussed above, anionic and cationic surfactants have been applied to glass surfaces. While most anionic surfactants are soluble in water, they do not form good stable coatings on glass in the presence of water. As shown by the data presented below, these surfactants were found unsuitable for use in protecting LCD glass.
The adsorption of cationic surfactants onto silica in solutions has been studied by a number of investigators in connection with understanding the dispersion of colloidal silica. See Goloub, T. P., Koopal, L. K., Bijsterbosch, B. H. Langmuir 1986, 12, 3188-3194; Goloub, T. P., Koopal, L. K., Langmuir 1997, 13, 673-681; Zajac, J., Tompette, J. L., Partyka, S., Langmuir 1996, 12, 1357-1367; Rosen, M. J. Surfactants and Interfacial Phenomena, J. Wiley and Sons, New York, 1989, Chapter 2; and Harell, J. H., Scamehorn, J. F. xe2x80x9cAdsorption from Mixed Surfactant Systemsxe2x80x9d, in Mixed Surfactant Systems, Surfactant Series Vol. 46, Ogino, K, and Abe, M, Ed.; Marcel Dekker, Inc. New York, 1992, pp. 263-281. At neutral pH, silicate surfaces are usually negatively charged allowing the ready adsorption of cationic species. The application of a cationic surfactant to glass at a temperature below the boiling point of water has been disclosed in Evans, U.S. Pat. No. 4,544,395.
As described in detail below, in accordance with the present invention, it has been discovered that cationic surfactants of various types (as well as non-ionic surfactants and betaines) are able to rapidly organize on hot glass (i.e., on glass having a temperature above 175xc2x0 C.) and form a coating which is sufficiently hydrophobic (i.e., has a sessile drop contact angle of at least 40xc2x0) to substantially reduce adhesion of glass chips to the glass (e.g., to reduce adhesion of glass chips of a size greater than 1 micron by at least 80%). The prior work involving cationic surfactants and glass do not disclose or suggest this important result.
To summarize, there has been a need in the art for a method for protecting glass articles, specifically, sheets of LCD glass, which has the following characteristics:
(1) the method must be one that can be readily incorporated in the overall glass forming process, specifically, at the end of the forming process, so that newly formed glass is protected substantially immediately after it is produced; among other things, to meet this criterion, the coating material must be (a) able to withstand the environment (e.g., high temperatures) of a glass forming line and (b) the method of applying the material must be safe for use in such an environment;
(2) the coating must be sufficiently hydrophobic to protect the glass from chip adhesion resulting from cutting and/or grinding of the glass sheet, as well as the adhesion of other contaminants, e.g., particles, that the glass may come into contact with during storage and shipment prior to use;
(3) the coating must be sufficiently robust to continue to provide protection after being exposed to substantial amounts of water during the cutting and/or grinding process;
(4) the coating must be substantially completely removable from the glass prior to its ultimate use in, for example, producing a liquid crystal display; and
(5) the coating must be composed of a material which can be tolerated in low levels in the ultimate use of the glass.
The present invention addresses and satisfies this long standing need in the art.
The present invention provides methods for temporarily protecting a surface of a glass article from ambient dirt and methods for temporarily protecting a surface of a glass article from chip adhesions.
In accordance with a first of its aspects, the invention provides a method for temporarily protecting a surface of a glass article by coating the surface with a removable hydrophobic film (also referred to herein as a xe2x80x9chydrophobic coatingxe2x80x9d or simply a xe2x80x9ccoatingxe2x80x9d). The film can have a thickness of from about one molecule to about ten molecules or, if desired, can have a thickness greater than ten molecules.
In accordance with a second of its aspects, the invention provides a method of reducing chip adhesions when making a cut or ground glass article, including (A) forming a stable hydrophobic film on the surface of the article; (B) cutting or grinding the glass article; and (C) removing the film. Again, the film can have a thickness of from about one molecule to about ten molecules or, if desired, can have a thickness greater than ten molecules.
In accordance with both of these aspects, the invention provides a method for treating glass having at least one substantially flat surface comprising:
(a) forming a hydrophobic coating on the surface by applying an aqueous solution comprising at least one surfactant to the surface as part of the manufacturing process for the glass, wherein:
(1) the manufacturing process produces newly formed glass at an elevated temperature;
(2) the newly formed glass is at a temperature above 175xc2x0 C. (preferably above 200xc2x0 C.) when it first comes into contact with the aqueous solution;
(3) the surfactant is selected from the group consisting of cationic surfactants, non-ionic surfactants, and betaines; and
(4) the coating reduces adhesion of glass chips to the surface;
(b) cutting the glass;
(c) grinding and/or polishing at least one edge of the cut glass; and
(d) removing the coating from the surface; wherein:
(i) water or a water-containing solution is applied to the coated surface during at least one of steps (b) and (c); and
(ii) the coating has a sessile drop contact angle of at least 40xc2x0 after steps (b) and (c).
In certain preferred embodiments, the coating is applied by spraying onto the hot glass. Other approaches for applying the coating can be used, e.g., dipping, meniscus coaters, wick coaters, etc., but are less preferred since the hot glass can often exhibit substantial back-and-forth movement at the end of the glass manufacturing process, especially, when a overflow downdraw process is used.
In other preferred embodiments, the coating is removed using an aqueous detergent solution, e.g., a commercial detergent package, preferably in combination with brush washing and/or ultrasonic cleaning. In addition, the surface of the coating can be exposed to an oxidizing atmosphere prior to being contacted with the detergent solution. The oxidizing atmosphere/detergent solution combination is preferably combined with brush washing and/or ultrasonic cleaning. The oxidizing atmosphere can be produced by corona discharge, through the use of UV light to produce ozone, or by means of an oxygen plasma. Ozonated water can also be used. Although less preferred, the oxidizing atmosphere approach by itself can be used to remove the coating.
Other aspects of the invention are described in detail below.